1. Field of Invention
The present invention relates to an isolated power converter circuit and a control method thereof; particularly, it relates to such isolated power converter circuit and control method thereof wherein power consumption in a stand-by mode is reduced.
2. Description of Related Art
FIG. 1A shows a schematic diagram of an isolated power converter circuit 100. As shown in FIG. 1A, a voltage Vac is for example an AC voltage signal, which is to be rectified; however, the voltage Vac can be a DC voltage signal. A rectifier circuit 101 rectifies the voltage Vac to generate an input voltage Vin. The rectifier circuit 101 for example is a bridge rectifier circuit. A transformer circuit 102 in the isolated power converter circuit 100 receives the input voltage Vin and converts it to an output voltage Vout. The isolated power converter 100 includes the aforementioned transformer circuit 102, a power switch 103, a control circuit 105, a current sense circuit 106, and a voltage sense circuit 107. The control circuit 105 generates a driving signal GATE according to a current sense signal CS generated by the current sense circuit 106 and a voltage sense signal COMP generated by the voltage sense circuit. The driving signal GATE operates the power switch 103 to convert the input voltage Vin to the output voltage Vout. The transformer circuit 102 includes a first winding W1, a second winding W2, and a third winding W3. The second winding W2 is coupled to a ground level. The first winding W1 and the third winding W3 are coupled to a reference level. The third winding W3 senses the output voltage Vout generated by the second winding W2, and the voltage sense circuit 107 generates the voltage sense signal COMP according to a voltage across the third winding W3.
FIG. 2A shows a schematic diagram of another isolated power converter circuit 200. A control circuit 205 generates the driving signal GATE according to the current sense signal CS generated by the current sense circuit 106 and the voltage sense signal COMP, to control the conversion from the input voltage Vin to the output voltage Vout. This prior art isolated power converter circuit 200 shown in FIG. 2A is different from the prior art isolated power converter circuit 100 shown in FIG. 1A in the feedback mechanism for the information of the output voltage Vout. A voltage sense circuit 207 of the isolated power converter circuit 200 is coupled to an output terminal of the second winding W2 of the transformer circuit 202, which senses the output voltage Vout directly. An opto-coupler circuit 204 converts a sense signal of the output voltage Vout sensed by the voltage sense circuit 207 to an optical signal, and the optical signal is further converted to the voltage sense signal COMP. The voltage sense signal COMP is inputted to the control circuit 205.
Comparing the prior art isolated power converter circuits 100 and 200, the isolated power converter circuit 100 has a drawback of lower control accuracy because it indirectly detects the output voltage Vout, but an advantage of lower power consumption in the stand-by mode or in a light load condition (i.e., the output voltage Vout is maintained at some level and not decreased because of no or low power consumption) because it generates the voltage sense signal COMP by the transformer winding W3 and the voltage sense circuit 107. Referring to FIG. 1B for example, when the isolated power converter circuit 100 is in the stand-by mode, and the voltage Vac is in a range between 80V and 260V, the stand-by mode power consumption Pstb is only several milliwatts (mW). On the other hand, the isolated power converter circuit 200 shown in FIG. 2A has a relatively higher control accuracy because it directly detects the output voltage Vout, but consumes relatively higher power in the stand-by mode or the light load condition because it converts the detection result of the output voltage Vout to the voltage sense signal COMP by the opto-coupler circuit 204. Referring to FIG. 2B, when the isolated power converter circuit 200 is in the stand-by mode, and the voltage Vac is in a range between 90V and 390V, the stand-by mode power consumption Pstb is several tens of milliwatts (mW), which is much higher.
In the prior art isolated power converter circuit 200, a current flowing through the opto-coupler circuit 204 is relatively higher when the isolated power converter circuit 200 is in the stand-by mode or the light load condition, while the current is relatively lower when the isolated power converter circuit 200 is in a normal operation mode. FIG. 2C shows a characteristic curve of the relationship between the current flowing through the opto-coupler circuit 204 (i.e., the opto-coupler current Iopt) and the output voltage Vout. When the isolated power converter circuit 200 is starting up, or when a load circuit consumes power, the output voltage Vout is lower than a voltage level VL; in such conditions, the isolated power converter circuit 200 operates to deliver power to the load circuit (to increase the output voltage Vout), but the opto-coupler current Iopt is relatively lower, and the opto-coupler circuit 204 consumes relatively lower power. When the output voltage Vout is higher than the voltage level VL, the isolated power converter circuit 200 operates in the stand-by mode; in such condition, the isolated power converter circuit 200 is not required to deliver power to the load circuit, but the opto-coupler current Iopt is relatively higher, and the opto-coupler circuit 204 consumes relatively higher power. The above arrangement of the opto-coupler current Iopt is for properly starting the isolated power converter circuit 200 in the starting stage. More specifically, because the power supplied to the opto-coupler circuit 204 is from the output voltage Vout, and the output voltage Vout is low in the starting stage, it must be so arranged that the opto-coupler current Iopt is low when the output voltage Vout is low, and the opto-coupler current Iopt is high when the output voltage Vout is high, such that the isolated power converter circuit 200 is properly feedback controlled according to the opto-coupler current Iopt, and the control circuit 205 which receives the voltage sense signal COMP can distinguish corresponding conditions of the output voltage Vout.
In view of above, to overcome the drawbacks in the prior art shown in FIGS. 1A (poor control accuracy) and 1B (high power consumption in the stand-by mode), the present invention proposes an isolated power converter circuit and a control method thereof, which has a high control accuracy but low the power consumption in the stand-by mode.
For reference, U.S. application 2011/0018590 assigned to the same assignee is related to this invention.